CoaXPress Interface Moves to Machine Vision Mainstream

It may seem as if there are far too many camera interfaces, but they're not all alike. Some, like USB and FireWire, are built on general-purpose protocols. The newest machine vision-specific interface, CoaXPress (CXP), is based on a different idea. A camera and frame grabber incorporating this interface debuted at this year's Vision 2011 show. Added to what's already out there, these items make choice in CoaXPress components a reality for engineers designing industrial vision systems.

Adimec demonstrated its first CoaXPress camera with dual interfaces. Each of the two CoaXPress interfaces of the Q-4A180/CXP area scan camera supports the standard's CXP-6 configuration, resulting in a maximum interface speed of 12.5Gbps. The camera uses two standard coaxial cables, each with a maximum transmission rate of 6.25Gbps over 45 meters.

Enough vision hardware components incorporating the coax-based CoaXPress camera interface, such as the Radient eV-CXP frame grabber from Matrox Imaging, are now available to give engineers a choice.
Source: Adimec

Matrox Imaging announced its Radient eV-CXP, a CoaXPress frame grabber based on the company's Radient family. The Radient eV-CXP has four independent CoaXPress links. This makes it possible to perform simultaneous image capture from up to four cameras, each of which can run at different CoaXPress speeds. Alternatively, the frame grabber can capture images from a single camera that uses link aggregation to transmit image data at up to 25Gbps.

Before the show, BitFlow began shipping its new Karbon-CXP frame grabber, which achieves video acquisition speeds of up to 6Gbps and sends control commands and triggers at 20Mbps over a single 75-Ohm coax cable up to 135 meters. Other cameras and frame grabbers have been released by Adimec, Active Silicon, Silicon Software, and Imperx.

The serial, packet-based, high-speed CoaXPress protocol was designed to overcome some of the bandwidth and cabling restrictions of the venerable parallel Camera Link protocol. CoaXPress achieves a base speed of 3.125Gbps per cable for distances of 100 meters between camera and computer, or a maximum sustained speed of 6.25Gbps per cable for distances of up to 45 meters. That compares very favorably with Camera Link's cabling limitation of 10 meters and a 6Gbps maximum speed that requires two cables.

Aside from speed, the most important feature of CoaXPress is the fact that it runs on good old coax. Coax cable is already installed everywhere. It's cheap, reliable, well-known, understood, and reliable. Engineers can achieve higher speeds by simply using multiple cables in parallel, and the standard doesn't limit how many cables they can deploy in a configuration. Today, coax cable is also available in many different varieties and quality grades.

There's still a huge legacy of analog cameras in industrial vision systems throughout the world, and a huge number of coax cables are connecting them. More than half of machine vision installations in Japan, for instance, are still dominated by analog cameras and coax cabling. Though CoaXPress is, of course, not backward compatible to analog interfaces, some consider it a relatively easy, cost-effective upgrade path from a cabling standpoint for introducing high-resolution digital cameras. The overall cost for replacing analog camera systems with digital CoaXPress cameras and frame grabbers would be much lower than replacing them with Camera Link cameras and frame grabbers. The opportunity is definitely there to repurpose a coax-based infrastructure with very low installation costs while reducing system complexity and improving performance.

John, thanks for that feedback. The complexity of the proprietary Camera Link interface and its specialized cables is often cited as a major reason why builders of machine vision systems look elsewhere for camera interface solutions. CoaXPress is certainly a majority contender to fill that need.

Another benefit not previously discussed is that CoaXPress can be used with simple, low-cost coaxial rotary joints. Camera Link, on the other hand, with its 26 conductors requires complex and costly slip-rings. This is of particular interest to the military, where the use of 360 degree rotating cameras is often required. Examples include gun turrets or for homeland security harbor surveillance.

William, that's a good question. It's not at all obvious, but "machine vision" usually refers to inspection during incoming materials/components, assembly, and manufacturing using computer vision technology. That said, the security area is another application for computer vision, but the requirements in terms of resolution and speed are much lower there than they are for product inspection. Even within product inspection, resolutions and speeds needed rfange widely, depending non whether the device under inspection (DUI) is a soda pop bottle or a semiconductor wafer.

The claimed speeds are very impressive, but the application in machine vision is a bit unclear to me. The immediate application that I can imagine is for recording vehicle crashes for vehicle safety development. That has been a big user of fast video recording for quite a while, and always looking for faster speeds. Of course, that area is less constrained by budgets, so it has not been the one to drive prices down. I suppose that really fast video would also be useful in other applications, but not always cost competitive.

Good questions, Alex and Beth. Since the standard was only finalized in March, and there are currently only a handful of companies making products, it's too soon to tell how fast the adoption rate will be or how widespread CoaXPress will become. It is expected to be very popular in geographic areas such as Asia, where there are a lot of analog cameras installed using coax cables. Promoters also expect it to be adopted by at least some first-time users. As in so many aspects of manufacturing systems, which standards are adopted, and what types of configurations are built, depend on many different factors.

Critiques of the protocol are also somewhat varied, depending on what it's being compared against. For example, promoters of both GigE/GigE Vision and Camera Link HS, the replacement for Camera Link, point out that CoaXPress, while serial, is not a point-to-multipoint protocol, something needed in the growing number of multi-camera systems to reduce cabling needs and speed data processing. Aside from those, additional critiques concern the protocol's SERDES topology, and start getting a bit esoteric.

How widely deployed is CoaXPress at this point among machine vision vendors, and what is the competitive spin/marketing angle that those vendors who aren't adopting it are using to push back against it?

Good questions, Beth. Yes, twice the per-cable speeds over cheaper cabling are some of the main benefits compared to the industry veteran, Camera Link. Camera Link requires its own special type of cabling which is pretty pricey compared to the ubiquitous coax. But there's another. CoaXPress cabling can carry that twice-as-fast signal 45 meters vs Camera Link's 10 meters before a repeater is needed.

So the benefit of the CoaXPress interface is better bandwidth and the ability to run over older cabling? For engineers designing machine vision systems, what kind of benefits does this bring to their applications--faster speeds at less reconfiguration expense? Any thing beyond that?

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